PROJECT SUMMARY A third of the world?s population is chronically infected with the parasite Toxoplasma gondii. Parasite infection can present in the host or fetus with symptoms ranging from flu-like, retinochoroiditis, or congenital toxoplasmosis (restricted to the fetus), and in immunocompromised individuals can lead to death. Additionally, in the Unites States T. gondii is the second leading cause of food borne related deaths. Thus, there is a precedence to understand the dynamic cellular and molecular mechanisms of host defense that control T. gondii infections. In response to this infection, the host mounts a robust innate and adaptive immune response that work in concert with each other for host defense. The innate immune response requires a coordinated molecular and cellular response characterized by inflammatory monocytes, dendritic cells (DCs), macrophages and neutrophils that lead to a strong CD4+ TH1-derived IFN-? reaction that is essential for host defense. The T- box transcription factor T-bet, encoded by Tbx21, is considered the master regulator of TH1-derived IFN-? response. However, our own work has unexpectedly demonstrated that T-bet is dispensable for parasite- mediated IFN-? production. However, T-bet-independent IFN-? production is insufficient for host protection and similar to IFN-? deficiency, T-bet-deficient (Tbx21-/-) mice rapidly succumb to T. gondii infection. Conversely, mice lacking T cells, but retaining a completely functional myeloid innate immune system survive significantly longer than infected Tbx21-/- mice. Furthermore, our data reveals that T-bet-independent IFN-? is sufficient to trigger IFN-mediated antimicrobial gene expression, which is indispensable for host survival during parasite infection. We go on to show that at day 8 post-infection there is a significant increase of wild-type T-bet expressing monocytes, DCs, macrophages, and neutrophils. Hence, our data suggests that T-bet expression within myeloid cells plays a significant role in host defense against the parasite. We hypothesize that T-bet deficiency within myeloid innate immune cells during acute T. gondii infection results in rapid host mortality. Within this proposal we plan to 1) establish that T-bet expression in myeloid innate immune cells is essential for host defense (Aim 1) and 2) define T-bet?s role in determining monocytes, DCs, macrophages, and neutrophils effector functions for parasite clearance and survival (Aim 2). This proposal will formally address T- bet-dependent effector functions in myeloid innate immune cells during parasite infection, establishing a new role for T-bet as a regulator of type I innate effector function for host defense. We believe that our proposal has much broader implications as to T-bet?s function not only for defense against T. gondii, but also for host survival against other type I intracellular pathogens such as Mycobacteium, Leishmania, Listeria, and Salmonella, which will open new avenues of research of T-bet-dependent effector function in myeloid innate immune cells.